/* prvy proces */
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define MEM_SZ 4096

struct shared_use_st {
  int written_by_you;
  char some_text[BUFSIZ];
};

int main (int argc, char * argv[]) {
  int running = 1;
  void *shared_memory = (void *) 0;
  struct shared_use_st *shared_stuff;
  int shmid;

  srand ((unsigned int) getpid ());

  shmid = shmget ((key_t) 1234, MEM_SZ, 0666 | IPC_CREAT);

  if (shmid == -1) {
    fprintf (stderr, "shmget failed\n");
    exit (EXIT_FAILURE);
  }

  shared_memory = shmat (shmid, (void *) 0, 0);
  if (shared_memory == (void *) -1) {
    fprintf (stderr, "shmat failed\n");
    exit (EXIT_FAILURE);
  }

  printf ("Memory attached at %X\n", shared_memory);

  shared_stuff = (struct shared_use_st *) shared_memory;
  shared_stuff->written_by_you = 0;
  while (running) {
    if (shared_stuff->written_by_you) {
      printf ("You wrote: %s", shared_stuff->some_text);
      sleep (rand () % 4);    /* make the other process wait for us ! */
      shared_stuff->written_by_you = 0;
      if (strncmp (shared_stuff->some_text, "end", 3) == 0) {
        running = 0;
      }
    }
  }

  if (shmdt (shared_memory) == -1) {
    fprintf (stderr, "shmdt failed\n");
    exit (EXIT_FAILURE);
  }

  if (shmctl (shmid, IPC_RMID, 0) == -1) {
    fprintf (stderr, "shmctl(IPC_RMID) failed\n");
    exit (EXIT_FAILURE);
  }

  exit (EXIT_SUCCESS);
}

/* druhy proces */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define MEM_SZ 4096

struct shared_use_st
{
  int written_by_you;
  char some_text[BUFSIZ];
};

int main (int argc, char * argv[]) {
  int running = 1;
  void *shared_memory = (void *) 0;
  struct shared_use_st *shared_stuff;
  char buffer[BUFSIZ];
  int shmid;

  shmid = shmget ((key_t) 1234, MEM_SZ, 0666 | IPC_CREAT);

  if (shmid == -1) {
    fprintf (stderr, "shmget failed\n");
    exit (EXIT_FAILURE);
  }

  shared_memory = shmat (shmid, (void *) 0, 0);
  if (shared_memory == (void *) -1) {
    fprintf (stderr, "shmat failed\n");
    exit (EXIT_FAILURE);
  }

  printf ("Memory attached at %X\n", shared_memory);

  shared_stuff = (struct shared_use_st *) shared_memory;
  while (running) {
    while (shared_stuff->written_by_you == 1) {
      sleep (1);
      printf ("waiting for client...\n");
    }
    printf ("Enter some text: ");
    fgets (buffer, BUFSIZ, stdin);

    strcpy (shared_stuff->some_text, buffer);
    shared_stuff->written_by_you = 1;

    if (strncmp (buffer, "end", 3) == 0) {
      running = 0;
    }
  }

  if (shmdt (shared_memory) == -1) {
    fprintf (stderr, "shmdt failed\n");
    exit (EXIT_FAILURE);
  }
  exit (EXIT_SUCCESS);
}